Embodiments of the invention hereinafter described pertain to devices for the generation of Karman vortices.
It is well known that a fluctuation flow field composed of vortices is present in the wake of an object located in a relatively moving fluid. That is, if an object is passing through a fluid, or if the fluid is passing an object, a fluctuating flow field is developed which is composed of vortices which are alternately shed from the sides of the object. This fluctuating flow field is more commonly referred to as a Karman vortex street or trail. As is also known, the rate of generation of vortices in the wake of the object is related to the relative velocity between the object and the fluid stream.
Vortex generators heretofore known in the art include objects having circular, triangular, or rectangular cross-sections. Generally a vortex generator cross-section has a dimension which must be sufficiently large or blunt with respect to one or more other dimensions of the generator in order to produce a stable vortex street. Similarly, when existing vortex generators are located in a confined fluid channel, such as a pipe or the like, often a stable vortex street is produced only when the generator creates a prescribed blockage ratio with respect to the channel. For example, some vortex generators currently marketed preferably operate when blocking 30% of the cross-sectional area of a flow channel.
Unfortunately, while existing vortex generators are thought to be designed to produce as stable a vortex street as possible, the degree of vortex stability and the related accuracy of measurement is inherently limited by the geometrical configurations of the generators employed. In several respects the particular configurations of existing generators create disturbances which result in inaccurate measurements.
In the above regard, it is quite well known that a stagnation region develops on an upstream surface of an object immersed in a relatively moving fluid. The stagnation region constitutes a pressure zone near the upstream surface. The size of the stagnation region, and the magnitude of pressure associated therewith, depends on the configuration of the upstream surface. As the fluid moves relative to the object, the stagnation region tends to shift from side-to-side about the upstream surface, thereby creating pressure differentials which, in turn, effect flow-induced pressure gradients occurring downstream along surfaces of the object. Since the point of detachment of vortices from the immersed object and the timing of the detachment is dependent upon these pressure gradients, the magnitude of the pressure differentials associated with the shifting of the stagnation region is significant. The stagnation region created by the relatively blunt upstream surfaces of existing generators precipitate pressure differentials which cause non-uniformity in the detachment of vortices from the generator. Such non-uniformity results in erroneous measurement of the rate of vortex generation.
The geometrical configurations of existing vortex generators also produce large, turbulent wakes. The more turbulent the wake the greater the pressure drag exerted on the generator and the fluid. The pressure drag undesirably effects the velocity of the fluid and the vortices generated at a rate proportional thereto. Thus, the turbulence caused by existing vortex generators has detrimental impact upon the flow measurements.
Various types of existing flow meters, including flow meters for measuring velocity and/or mass of fluid flow, utilize vortex generating devices to obtain flow measurements. However, many of the vortex generating devices are unable to produce a vortex street of sufficient stability through a wide range of fluid flow rates. As a result, measurements indicate no vortices, or "misses", where vortices should have been detected. (Applicants recognize that vortex "stability" is dependent upon both the strength of formation of the vortices and the sensitivity of the means used for detection of the vortices. As used herein, however, "stability" pertains to the quality of vortices with reference to a given, or constant, detection means. That is, the production of sufficiently well-formed vortices in a sequence indicative of fluid flow.) The instability of the vortex streets produced by existing generators requires that vortex counting measurements be averaged over a sufficiently long time period in order to obtain an accurate indication of flow rate. The longer time interval required for averaging, results in a less accurate measurement for instantaneous flow.
Therefore, an object of embodiments of the invention hereinafter described is the provision of a device for generating stable vortices.
An advantage of these the embodiments is the reliable production of stable vortices without a significant non-flow induced pressure differential in the region of the vortex generating device.
A further advantage associated with the embodiments of the invention is the accurate measurement of the rate of generation of Karman vortices.